CMP abrasive, liquid additive for CMP abrasive and method for polishing substrate

ABSTRACT

A CMP abrasive comprising a cerium oxide slurry containing cerium oxide particles, a dispersant and water, and a liquid additive containing a dispersant and water; and a liquid additive for the CMP abrasive. A method for polishing a substrate which comprises holding a substrate having, formed thereon, a film to be polished against a polishing pad of a polishing platen, followed by pressing, and moving the substrate and the polishing platen while supplying the above CMP abrasive in between the film to be polished and the polishing pad to thereby polish the film to be polished. The CMP abrasive and the method for polishing can be used for polishing a surface to be polished such as a silicone oxide film or a silicon nitride film without contaminating the surface to be polished with an alkali metal such as sodium ions and with no flaws, and the CMP abrasive is excellent in storage stability.

This application is a divisional of application Ser. No. 11/177,352,filed Jul. 11, 2005 which is a continuation of application Ser. No.10/990,427, filed Nov. 18, 2004, which is a continuation of applicationSer. No. 10/759,163, filed Jan. 20, 2004 (now abandoned), which is aDivisional of application Ser. No. 09/856,491 filed Jun. 19, 2001 (nowU.S. Pat. No. 6,783,434), which is a national stage application filedunder 35 U.S.C. § 371 of International Application No. PCT/JP99/07209,filed Dec. 22, 1999, the entire disclosures of which are herebyincorporated by reference.

TECHNICAL FIELD

The invention relates to a CMP abrasive usable in the production ofsemiconductor elements, a liquid additive for CMP abrasive and a methodfor polishing substrates. Particularly, it relates to a CMP abrasiveusable in the step of planarizing the surface of a substrate, typicallythe steps of planarizing an interlayer insulating film and formingshallow-trench separation, to a liquid additive for the CMP abrasive andto a method for polishing substrates by using the CMP abrasive.

BACKGROUND ART

In present ultra large scale integrated circuits, packaging density isincreasing, and various fine processing techniques have been studied anddeveloped. Design rules have already reached the order of sub halfmicron. CMP (Chemical Mechanical Polishing) is one of the techniquesdeveloped to satisfy such a strict requirement for fineness. Thistechnique is essential for the production of semiconductor devices,typically for planarizing interlayer insulating films and forshallow-trench separation, because it can completely planarize layers tobe exposed, reducing the burden on exposure techniques and stabilizingthe production yield.

Colloidal silica abrasives have been investigated as common CMPabrasives to be used in the production of semiconductor devices toplanarize inorganic insulating films, such as silicon oxide insulatingfilm, formed by plasma-CVD (Chemical Vapor Deposition), low pressure-CVDor the like. Colloidal silica abrasives may be produced by using silicaparticles, which are typically formed from tetrachlorosilane throughthermal decomposition, and adjusting pH. Such abrasives, however, cannotpolish inorganic insulating films fast enough, and need higher polishingrate for their practical use.

In integrated circuits with design rules of 0.5 μm or more, devices wereseparated by LOCOS (Localized Oxidation of Silicon). As the processingmeasurements have become finer, shallow-trench separation has becomeused in response to the requirement for a technique giving narrowerseparation gap between devices. For shallow-trench separation, thesurplus parts of a silicon oxide film formed on a substrate are removedby CMP, and a stopper film reducing the polishing rate is provided underthe silicon oxide film to stop polishing. The stopper film is typicallymade of silicon nitride, and the rates of polishing the silicon oxidefilm and the stopper film are preferably in a large ratio. Whereconventional colloidal silica abrasives are used, the ratio between therate of polishing the silicon oxide film and the rate of polishing thestopper film is as small as the order of 3, and such abrasives cannotsatisfy the requirements of practical shallow-trench separation.

On the other hand, cerium oxide abrasives have been used for polishingphoto masks or the surface of glass, such as lenses. Having lowerhardness as compared to silica particles and alumina particles, ceriumoxide particles hardly make flaws on the polished surface and aresuitable for finish mirror polishing. However, the cerium oxideabrasives for polishing glass surfaces cannot be used as abrasives forpolishing semiconductors, because they contain a dispersant containingsodium salts.

DISCLOSURE OF INVENTION

An object of the invention is to provide a CMP abrasive, which canspeedily polish a surface to be polished, such as a silicon oxideinsulating film, without making flaws.

Another object of the invention is to provide a CMP abrasive, which canspeedily polish a surface to be polished, such as a silicon oxideinsulating film, without contaminating the surface to be polished withalkali metals, such as sodium ions, nor making flaws.

Another object of the invention is to provide a CMP abrasive, which ismore advantageous in that it can increase the ratio of the rate ofpolishing a silicon oxide insulating film to the rate of polishing asilicon nitride insulating film.

Another object of the invention is to provide a CMP abrasive, which canspeedily polish a surface to be polished, such as a silicon oxideinsulating film, without contaminating the surface to be polished withalkali metals, such as sodium ions nor making flaws and contains acerium oxide slurry improved in storage stability.

Another object of the invention is to provide a CMP abrasive, which canspeedily polish a surface to be polished, such as a silicon oxideinsulating film, without contaminating the surface to be polished withalkali metals, such as sodium ions, nor making flaws, and can increasethe ratio of the rate of polishing a silicon oxide insulating film tothe rate of polishing a silicon nitride insulating film to 50 or more.

Another object of the invention is to provide a liquid additive, whichis to be used to give a CMP abrasive improved in storage stability.

Another object of the invention is to provide a liquid additive for CMPabrasive to be used to improve the flatness of the polished surface of asubstrate.

Another object of the invention is to provide a method for polishing asubstrate, which can polish a surface of the substrate without makingflaws on its polished surface.

Another object of the invention is to provide a method for polishing asubstrate, which can speedily polish a surface to be polished, such as asilicon oxide insulating film, without making flaws, and can increasethe ratio of the rate of polishing a silicon oxide insulating film tothe rate of polishing a silicon nitride insulating film to 50 or more.

Accordingly, the invention relates to:

(1) a CMP abrasive comprising a cerium oxide slurry containing ceriumoxide particles, a dispersant and water; and

a liquid additive containing a dispersant and water;

(2) the CMP abrasive of (1), wherein each of the dispersants containedin the cerium oxide slurry and the liquid additive respectively is apolymer dispersant, which is a polymer containing ammonium acrylate as acopolymerized ingredient;

(3) the CMP abrasive of (1), wherein each of the dispersants containedin the cerium oxide slurry and the liquid additive respectively is apolymer dispersant, which is a polyammonium-acrylate or apolyamine-acrylate;

(4) the CMP abrasive of (2) or (3), wherein the polymer dispersants havea weight average molecular weight of 100 to 50,000;

(5) the CMP abrasive of (1), wherein the cerium oxide slurry contains0.01 to 2.0 parts by weight of the dispersant relative to 100 parts byweight of the cerium oxide particles and contains 0.3 to 40% by weightof the cerium oxide particles based on the cerium oxide slurry;

(6) the CMP abrasive of any one of (1) to (5), wherein the cerium oxideslurry is pH 6 to 10;

(7) the CMP abrasive of any one of (1) to (6), which is 50 or more inratio of rate of polishing a silicon oxide film to rate of polishing asilicon nitride film;

(8) a liquid additive for CMP abrasive comprising a dispersant andwater;

(9) the liquid additive for CMP abrasive of (8), which contains 1 to 10%by weight of the dispersant;

(10) the liquid additive for CMP abrasive of (9), wherein the dispersantis a polyammonium-acrylate or a polyamine-acrylate;

(11) the liquid additive for CMP abrasive of (10), wherein thepolyammonium-acrylate or the polyamine-acrylate has a weight averagemolecular weight of 1,000 to 100,000;

(12) the liquid additive for CMP abrasive of (11), wherein thepolyammonium-acrylate or the polyamine-acrylate has a molecular weightdistribution (weight average molecular weight/number average molecularweight) of 1.005 to 1.300;

(13) the liquid additive for CMP abrasive of (10), wherein thepolyammonium-acrylate or the polyamine-acrylate contains 10 mol % orless of free ammonia or a free amine, which does not form a salt;

(14) the liquid additive for CMP abrasive of (10), which is pH 4 to 8;

(15) the liquid additive for CMP abrasive of (10), which has a viscosityof 1.20 to 2.50 mPa·s;

(16) a method for polishing a substrate, comprising holding a substratehaving, formed thereon, a film to be polished against a polishing pad ofa polishing platen, followed by pressing, and moving the substrate andthe polishing platen while supplying the CMP abrasive of any one of (1)to (7) in between the film to be polished and the polishing pad tothereby polish the film to be polished; and

(17) the method of (16), wherein the substrate to be polished has atleast a silicon oxide film or a silicon nitride film formed thereon.

BEST MODE FOR CARRYING OUT THE INVENTION

Cerium oxide may be produced by the oxidation of a cerium compound, suchas carbonate, nitrate, sulfate or oxalate of cerium. Conventional ceriumoxide abrasives for polishing silicon oxide films formed by TEOS-CVD orthe like contain monocrystalline cerium oxide particles with largeprimary particle sizes, and tend to make polishing flaws. Therefore, thecerium oxide particles to be used in the invention are not limited inthe method of production, but are preferably polycrystals that areaggregates of monocrystals of 5 nm to 300 nm. For polishingsemiconductor chips, the cerium oxide particles preferably contain aslittle as 10 ppm or less of alkali metals and halogens.

The methods usable in the invention to produce the cerium oxideparticles include burning or oxidation using hydrogen peroxide or thelike. Preferred burning temperatures range from 350 to 900° C. The rawmaterial suitable for the method is cerium carbonate.

The above method gives aggregates of cerium oxide particles, which arethen preferably pulverized mechanically. Examples of preferredpulverizing methods include dry grinding using a jet mill, and wetgrinding using a planetary bead mill.

The cerium oxide slurry to be used in the invention is obtainable, forexample, by dispersing a composition comprising the cerium oxideparticles having the above characteristics, a dispersant for dispersingthe cerium oxide particles in water and water. The content of the ceriumoxide particles is not limited but is preferably 0.3 to 40% by weight,more preferably 0.5 to 20% by weight to handle the dispersion easily. Inthe CMP abrasive obtainable by mixing the cerium oxide slurry and aliquid additive, the content of the cerium oxide particles is preferably0.01 to 10% by weight, more preferably 0.1 to 5% by weight.

The dispersant to be used in the invention comprises one or twocompounds selected from polymer dispersants, water-soluble anionicsurfactants, water-soluble nonionic surfactants, water-soluble cationicsurfactants and water-soluble amphoteric surfactants. For polishingsemiconductor chips, the dispersant preferably contains as little as 10ppm or less of alkali metals, such as sodium ions and potassium ions,halogens and sulfur.

Examples of polymer dispersants include polymers of unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid or maleic acid,or ammonium salts or amine salts of the polymers; copolymers of anunsaturated carboxylic acid, such as acrylic acid, methacrylic acid ormaleic acid, with a copolymerizable monomer, for example, an alkylacrylate, such as methyl acrylate or ethyl acrylate, a hydroxyalkylacrylate, such as hydroxyethyl acrylate, an alkyl methacrylate, such asmethyl methacrylate or ethyl methacrylate, a hydroxyalkyl methacrylate,such as hydroxyethyl methacrylate, vinyl acetate or vinyl alcohol, andammonium salts or amine salts of the copolymers. The unsaturatedcarboxylic acid moieties in the polymers or copolymers may be convertedinto ammonium salts either before or after polymerization. The polymersand copolymers preferably contain 1 to 100 mol %, more preferably 10 to100 mol % of unsaturated carboxylic acid moieties.

Preferred dispersants are polymers containing ammonium acrylate as acopolymerized ingredient, polyammonium-acrylates andpolyamine-acrylates. Polyammonium-acrylates and polyamine-acrylatespreferably have weight average molecular weights of 1,000 to 100,000,more preferably 3,000 to 60,000, further preferably 10,000 to 40,000. Ifthe weight average molecular weight is less than 1,000, cerium oxideparticles may aggregate, and if more than 100,000, polishing rate ratiomay be reduced. The polyammonium-acrylates and polyamine-acrylatespreferably have molecular weight distributions (weight average molecularweight/number average molecular weight) of 1.005 to 1.300, morepreferably 1.100 to 1.250, further preferably 1.150 to 1.200. If themolecular weight distribution is less than 1.005, the cerium oxideparticles may aggregate, and if more than 1.300, the polishing rateratio may be reduced. Herein, the weight average molecular weight andnumber average molecular weight are measured through gel permeationchromatography, based on the calibration curve of a standard,polystyrene.

The polyammonium-acrylates and polyamine-acrylates are obtainablethrough neutralization of a mixture comprising a polyacrylic acid and anequimolar amount of ammonia or an amine relative to carboxyl groups,and, in view of high flatness, preferably contain as little as 10 mol %or less of free ammonia or amine forming no salts (that is, at least 90mol % of the carboxyl groups of polyacrylic acid are preferablyneutralized). The amount of the free ammonia or amine forming no saltscan be determined by adding an organic solvent to precipitate thepolymer, filtering the polymer and quantitatively determining the amountof ammonia or amine in the filtrate.

Examples of water-soluble anionic surfactants include triethanolaminelauryl sulfate, ammonium lauryl sulfate and triethanolaminepolyoxyethylene alkyl ether sulfates.

Examples of water-soluble nonionic surfactants include polyoxyethylenelauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearylether, polyoxyethylene oleyl ether, polyoxyethylene higher alcoholethers, polyoxyethylene octyl phenyl ether, polyoxyethylene nonyl phenylether, polyoxyalkylene alkyl ethers, polyoxyethylene derivatives,polyoxyethylenesorbitan monolaurate, polyoxyethylenesolbitanmonoparmitate, polyoxyethylenesolbitan monostearate,plyoxyethylenesolbitan tristearate, polyoxyethylenesolbitan monooleate,polyoxyethylenesolbitan trioleate, polyoxyethylenesolbitol tetraoleate,polyethylene glycol monolaurate, polyethylene glycol monostearate,polyethylene glycol distearate, polyethylene glycol monooleate,polyoxyethylene alkyl amines, polyoxyethylene hardened castor oil andalkylalkanolamides. Examples of water-soluble cationic surfactantsinclude coconutamine acetate and stearylamine acetate.

Examples of water-soluble amphoteric surfactants include laurylbetaine,stearylbetaine, lauryldimethylamine oxide and2-alkyl-N-carboxymethyl-N-hydroxyethylimidazoliniumbetaine.

In view of the dispersibility of particles in the cerium oxide slurry,the prevention of their sedimentation and the relationship betweenpolishing flaws and the amount of the dispersants added, the amount ofthe dispersant added in the cerium oxide slurry is preferably 0.01 to2.0 parts by weight relative to 100 parts by weight of cerium oxideparticles.

Among the dispersants described above, the polymer dispersantspreferably have weight average molecular weights of 100 to 100,000, morepreferably 100 to 50,000, further preferably 1,000 to 10,000, asmeasured through gel permeation chromatography based on the calibrationcurve of the standard, polystyrene. If the molecular weight of thedispersant is too low, silicon oxide films or silicon nitride films maynot be polished speedily enough, and if it is too high, the cerium oxideslurry may be too viscous and lose storage stability.

The pH of the cerium oxide slurry is preferably 6 to 10. If the pH istoo low, a liquid mixture of the cerium oxide slurry and a liquidadditive may lose storage stability and make polishing flaws on polishedsilicon oxide or silicon nitride films, and if the pH is too high, aliquid mixture of the cerium oxide slurry and a liquid additive may alsolose storage stability and make polishing flaws on polished siliconoxide or silicon nitride films. The pH may be adjusted by adding anaqueous ammonia and stirring.

The cerium oxide particles can be dispersed in water by using a commonstirrer or others, such as a homogenizer, an ultrasonic disperser or awet ball mill.

The cerium oxide particles in the slurry thus produced preferably havean average particle size of 0.01 to 1.0 μm. This is because cerium oxideparticles with a too small average particle size have a considerably lowpolishing rate, and that with a too large average particle size tends tomake flaws on the polished film.

The liquid additive for CMP abrasive of the invention comprises adispersant and water. The dispersant is for dispersing the cerium oxideparticles contained in the above-described cerium oxide slurry in water.In view of the polishing rate ratio and the high flatness of thepolished surface, the dispersants suitable for the cerium oxide slurryare also suitable for the liquid additive. The dispersants used in thecerium oxide slurry and the liquid additive may be identical with ordifferent from each other. The concentration of the dispersant in theliquid additive is preferably 1 to 10% by weight. If it is less than 1%by weight, the polished surface may be less flat, and if more than 10%by weight, the cerium oxide particles may aggregate.

The CMP abrasive of the invention is used so that the cerium oxideslurry and the liquid additive prepared apart from each other are mixedat the time of polishing. If the cerium oxide slurry and the liquidadditive are stored in a form of a mixture, the cerium oxide particleswill aggregate, thereby making polishing flaws and causing a change inpolishing rate. Therefore, the liquid additive and the cerium oxideslurry are supplied on a polishing platen separately and mixed thereon,or mixed immediately before polishing and then supplied onto a polishingplaten. The mixing ratio between the cerium oxide slurry and the liquidadditive is not particularly limited so far as the desiredconcentrations are finally given.

In view of the dispersibility of particles in the slurry, the preventionof their sedimentation and the relationship between polishing flaws andthe amount of the dispersant added, the amount of the dispersant in theliquid additive relative to the cerium oxide is preferably 0.001 to 2000parts by weight, more preferably 0.01 to 1000 parts by weight, furtherpreferably 0.01 to 500 parts by weight, relative to 100 parts by weightof the cerium oxide particles in the cerium oxide slurry.

The specific gravity of the liquid additive is preferably 1.005 to1.050, more preferably 1.007 to 1.040, further preferably 1.010 to1.030. If the specific gravity is less than 1.005, the polished surfacemay be less flat, and if more than 1.050, the cerium oxide particles mayaggregate. The pH of the liquid additive is preferably 4 to 8, morepreferably 5 to 7, further preferably 6 to 7. If the pH is less than 4,the polishing rate may be reduced, and if more than 8, the polishedsurface may be less flat. The adjustment of pH may be performed byadding an acid or an alkali, such as acetic acid or aqueous ammonia, tothe liquid additive. The viscosity of the liquid additive at 25° C. ispreferably 1.20 to 2.50 mPa·s, more preferably 1.30 to 2.30 mPa·s,further preferably 1.40 to 2.20 mPa·s. If the viscosity is less than1.20 mPa·s, the cerium oxide particles may aggregate, and if more than2.50 mPa·s, the polished surface may be less flat.

In the CMP abrasive of the invention, the cerium oxide slurry and theliquid additive may be used as they are, or non-polymeric additives,such as N,N-diethylethanolamine, N,N-dimethylethanolamine oraminoethylethanolamine, may be added to the cerium oxide slurry or theliquid additive for CMP. The amounts of such additives are such thattheir total concentration in the resulting CMP abrasive is preferably0.001 to 20% by weight, more preferably 0.01 to 10% by weight.

The inorganic insulating film for which the CMP abrasive of theinvention is used may be formed, for example, by low pressure CVD orplasma CVD. To form a silicon oxide film by low pressure CVD,monosilane: SiH₄ is used as an Si-source, and oxygen: O₂ as anoxygen-source, and SiH₄—O₂ oxidation is carried out at a low temperatureof 400° C. or lower. After CVD, heat treatment at a temperature nothigher than 1000° C. may optionally be carried out. Where phosphorus: Pis doped to planarize the surface by high temperature reflow, anSiH₄—O₂—PH₃ reaction gas is preferably used. Plasma CVD is advantageousin that chemical reactions requiring high temperatures under normal,thermal equilibrium can undergo at lower temperatures. The methods forgenerating plasma include two types of capacitive coupling and inductivecoupling. Examples of reaction gases include an SiH₄—N₂O gas comprisingSiH₄ as an Si-source and N₂O as an oxygen-source; and a TEOS-O₂ gascontaining tetraethoxysilane (TEOS) as an Si-source (TEOS-plasma CVD).The preferred temperature of the substrate ranges from 250 to 400° C.,and preferred reaction pressure ranges from 67 to 400 Pa. As describedabove, the silicon oxide film to be used in the invention may be dopedwith other elements, such as phosphorus or boron.

To form a silicon nitride film by low pressure CVD, dichlorosilane:SiH₂Cl₂ is used as an Si-source, and ammonia: NH₃ as a nitrogen-source,and the SiH₂Cl₂—NH₃ oxidation is carried out at a high temperature of900° C. An example of the reaction gas for plasma CVD is an SiH₄—NH₃ gascomprising SiH₄ as an Si-source and NH₃ as a nitrogen-source. Thepreferred temperature of the substrate ranges from 300 to 400° C.

The substrate to be used may be a semiconductor substrate bearingcircuit devices and wiring patterns formed thereon, or a semiconductorsubstrate which bears circuit devices formed thereon and is furthercoated with a silicon oxide film layer or a silicon nitride film layer.Polishing a silicon oxide film layer or a silicon nitride film layerformed on such a semiconductor substrate by using the CMP abrasivesmoothes out the unevenness on the surface of the silicon oxide filmlayer, to planarize whole the surface of the semiconductor substrate. Itis also applicable for shallow-trench separation. Shallow-trenchseparation needs a ratio of the rate of polishing a silicon oxide filmto the rate of polishing a silicon nitride film (silicon oxidefilm-polishing rate/silicon nitride film-polishing rate) of 10 or more.If the ratio is too small, the difference between the silicon oxidefilm-polishing rate and the silicon nitride film-polishing rate will betoo small to stop polishing at a position predetermined forshallow-trench separation. If the ratio is 50 or more, polishing can bestopped easily by the further reduced polishing rate of silicon nitridefilm, and the CMP abrasive with such a ratio is more suitable forshallow-trench separation.

The polishing apparatus to be used may be a common one, which has aholder for holding a semiconductor substrate and a platen (equipped witha motor or the like capable of changing rotational speed) applied with apolishing pad. The material of the polishing pad may be any one, such asa non-woven fabric, a polyurethane foam or a porous fluorine resin. Thepolishing pad is preferably grooved to collect the CMP abrasive in thegrooves. The polishing conditions are not limited, but the rotationalspeed of the platen is preferably as low as 200 rpm or less to preventthe semiconductor substrate from being emitted. The pressure applied tothe semiconductor substrate is preferably 1 kg/cm² or less not to makepolishing flaws. For shallow-trench separation, polishing should makefew flaws. During polishing, the slurry is continuously supplied to thepolishing pad by some means, such as a pump. Not limitative butpreferred amount of the slurry supplied is such that the surface of thepolishing pad is always coated with the slurry.

After polishing, the semiconductor substrate is preferably washed wellin running water and then dried after blowing away the water dropletsfrom the semiconductor substrate by a spin drier or the like. Thus aplanarized shallow-trench structure is formed. Subsequently, aluminumwiring is formed on the silicon oxide insulating film layer, and asilicon oxide insulating film is again formed between and on the wiringby the same method as described above and polished by using the CMPabrasive to smooth out the unevenness on the insulating film surface,thereby planarizing whole the surface of the semiconductor substrate.The process is repeated to produce a semiconductor with desired layers.

The CMP abrasive of the invention can polish not only the silicon oxidefilm formed on a semiconductor substrate but also an inorganicinsulating film formed on a wiring board bearing a predetermined wiring,such as a silicon oxide film, glass or silicon nitride; optical glass,such as photo masks, lenses and prisms; inorganic conductor films, suchas ITO; optical integrated circuits, optical switching devices andoptical guides, which are made of glass and crystalline materials; theend faces of optical fibers; optical monocrystals, such asscintillators; solid-state laser monocrystals; sapphire substrates forblue laser LED; semiconductor monocrystals, such as SiC, GaP and GAS;glass substrates for magnetic discs; and magnetic heads.

Hereinafter, the invention will be described in more detail referring toExamples and Comparative Examples, which however do not limit the scopeof the invention.

Preparation 1 (Preparation of Cerium Oxide Particles)

2 kg of cerium carbonate hydrate was placed in a platinum vessel andburned in the air at 700° C. for 2 hours, to give about 1 kg ofyellowish white powder. The powder was identified to be cerium oxide byX-ray diffractiometry. The cerium oxide powder was mixed with deionizedwater to 10% by weight concentration, and pulverized with a horizontalwet ultrafine dispersing-pulverizer at 1400 rpm for 120 minutes. Theresulting liquid abrasive was heated to 110° C. for 3 hours to give drycerium oxide particles. The cerium oxide particles were polycrystalscomprising 10 to 60-nm-particle size primary particles as observed by atransmission electron microscope, and had a specific surface area of39.5 m²/g as measured by the BET method.

Preparation 2 (Preparation of Cerium Oxide Particles)

2 kg of cerium carbonate hydrate was placed in an platinum vessel andburned in the air at 700° C. for 2 hours, to give about 1 kg ofyellowish white powder. The powder was identified to be cerium oxide byX-ray diffractiometry.

1 kg of the cerium oxide powder was dry-ground with a jet mill. Thecerium oxide particles were polycrystals comprising 10 nm to60-nm-particle size primary particles as observed by a transmissionelectron microscope, and had a specific surface area of 41.2 m²/g asmeasured by the BET method.

Preparation 3 (Preparation of Cerium Oxide Slurry)

125 g of the cerium oxide particles prepared in Preparation 1, 3 g of a40-wt % aqueous solution of an ammonium salt of a polyacrylic acidcopolymer, which was a 3:1—copolymerization product of acrylic acid andmethyl acrylate and had an weight average molecular weight of 10,000,and 2372 g of deionized water were mixed, and ultrasonically dispersedwith stirring. The dispersing was conducted for 10 minutes with anultrasonic frequency of 40 kHz. The resulting slurry was filteredthrough a 0.8-μm filter, and deionized water was added thereto to give a2-wt % cerium oxide slurry (A-1). The pH of the cerium oxide slurry(A-1) was 8.5. The cerium oxide slurry (A-1) contained particles with anaverage particle size of as small as 0.20 μm as determined from theirparticle size distribution measured with a laser diffraction sizedistribution measuring apparatus. 95.0% of the particles were 1.0 μm orless.

Preparation 4 (Preparation of Cerium Oxide Slurry)

A cerium oxide slurry (A-2) was prepared in the same manner as inPreparation 3, except the cerium oxide particles prepared in Preparation1 were replaced by the cerium oxide particles prepared in Preparation 2.The pH of the cerium oxide slurry (A-2) was 8.7. The cerium oxide slurry(A-2) contained particles with an average particle size of as small as0.21 μm as determined from their particle size distribution. 95.0% ofthe particles were 1.0 μm or less.

Preparation 5 (Preparation of Cerium Oxide Particles)

2 kg of cerium carbonate hydrate was placed in a platinum vessel andburned in the air at 900° C. for 2 hours, to give about 1 kg ofyellowish white powder. The powder was identified to be cerium oxide byX-ray diffractiometry. 1 kg of the cerium oxide powder was dry-groundwith a jet mill. The cerium oxide particles were monocrystals of 80 to150 nm in particle size as observed by a transmission electronmicroscope, and had a specific surface area of 18.5 m²/g as measured bythe BET method.

Preparation 6 (Preparation of Cerium Oxide Slurry)

A cerium oxide slurry (B-1) was prepared in the same manner as inPreparation 3, except the cerium oxide particles prepared in Preparation1 were replaced by the cerium oxide particles prepared in Preparation 5.The pH of the cerium oxide slurry (B-1) was 8.4. The cerium oxide slurry(B-1) contained particles with an average particle size of as small as0.35 μm as determined from their particle size distribution. 85.5% ofthe particles were 1.0 μm or less.

EXAMPLES 1-10 AND COMPARATIVE EXAMPLES 1 AND 2

Cerium oxide slurries and liquid additives were prepared to prepare theCMP abrasives as shown in Table 1, and mixtures of a cerium oxide slurryand a liquid additive were used for polishing an insulating film in themanner described below. The results are listed in Table 1.

The liquid additive used in each of Examples 1-5, 7 and 9 was preparedby dissolving in deionized water a predetermined amount of the samedispersant as that used in the cerium oxide slurry of Example 1.

The dispersant used in Examples 6, 8 and 10 was a polyammonium-acrylatehaving a weight average molecular weight of 10,000, a number averagemolecular weight of 8,333, a molecular weight distribution of 1.2 and acontent of free ammonium of 4.3 mol %. The liquid additive used inExample 6 had a viscosity of 1.46 mPa·s and a specific gravity of 1.010.

In Comparative Example 2, the same cerium oxide slurry and liquidadditive as those used in Example 1 were previously mixed, and themixture was used for polishing an insulating film one day after.

(Polishing of Insulating Film)

A 125-mm-diameter silicon wafer with a silicon oxide film formed thereonby TEOS-plasma CVD was fixed to a holder to which an attraction pad forfixing substrates was bonded, and was then set, with the surface of theinsulating film directed downwardly, on a platen to which a polishingpad made of a porous urethane resin was bonded. A weight was then placedon it to produce a load of 300 g/cm². The insulating film was polishedby rotating the platen at 40 rpm for 2 minutes while feeding a ceriumoxide slurry (solid content: 2% by weight) and a liquid additiveseparately both at a rate of 25 ml/min and dropping them as one liquidonto the platen by controlling nozzles so that they joined together justabove the platen. After polishing, the wafer was removed from theholder, washed with running water well, and then with an ultrasoniccleaner for 20 minutes. After washing, water droplets were removed by aspin drier, and the wafer was dried for 10 minutes in a 120° C. drier.The change in the film thickness before and after polishing was measuredwith a photo-interferent film thickness measuring apparatus to determinethe polishing rate.

In place of the silicon oxide film formed by TEOS-plasma CVD, a siliconnitride film produced by low pressure CVD was polished in the samemanner under the same conditions, and the change in the film thicknessbefore and after polishing was measured, to determine the polishingrate. The results of the measurements of film thickness show that thesilicon oxide film produced by TEOS-plasma CVD and the silicon nitridefilm produced by low pressure CVD were made uniform in thickness allover the wafers. No flaws were observed on the surfaces of theinsulating films by visual observation under a mercury-vapor lamp, butthe surfaces were further observed precisely with an apparatus forexamining the appearance of wafers (trade name: OLYMPUS Al-2000,produced by Olympus Optical Co., Ltd.).

Similarly, a silicon oxide film having 20-μm-square 5,000-Å-highprojections at 100-μm distances was polished, and the degree of dishingwas measured at intermediate points between the polished projections toevaluate the flatness. TABLE 1-1 Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Cerium Name (A-1) (A-1) (A-1) (A-2) (A-2) (A-1)oxide Primary 10-60 10-60 10-60 10-60 10-60 10-60 slurry particle sizePolycrystal Polycrystal Polycrystal Polycrystal Polycrystal Polycrystal(500 g) (nm) pH 8.5 8.5 8.5 8.7 8.7 8.5 Liquid Dispersant Acrylic acid/Acrylic acid/ Acrylic acid/ Acrylic acid/ Acrylic acid/ Acrylic acid/additive methyl methyl methyl methyl methyl methyl (500 g) acrylate =3/1 acrylate = 3/1 acrylate = 3/1 acrylate = 3/1 acrylate = 3/1 acrylate= 10/0 Weight 10,000 10,000 10,000 10,000 10,000 10,000 averagemolecular weight Conc. (wt %) 1 2 6 2 2 3 pH 7.3 7.5 7.7 7.5 7.5 6.8Plasma-CVD-TEOS-silicon 2,000 2,000 1,500 2,000 2,000 1,800 oxide filmpolishing rate (Å/min) Low pressure-CVD- 40 20 20 20 20 20 siliconnitride film polishing rate (Å/min) Polishing rate ratio 50 100 75 100100 90 (silicon oxide film/ silicon nitride film) Flaws on polishedoxide 0.05 0.05 0.05 0.05 0.05 0.05 film (number/cm²) Degree of dishing(Å) 150 120 100 130 130 80

TABLE 1-2 Comparative Comparative Example 7 Example 8 Example 9 Example10 example 1 example 2 Cerium Name (B-1) (A-1) (A-1) (A-1) (A-1) (A-1)oxide Primary 80-150 10-60 10-60 10-60 10-60 10-60 slurry particle sizeMonocrystal Polycrystal Polycrystal Polycrystal Polycrystal Polycrystal(500 g) (nm) pH 8.4 8.5 8.5 8.5 8.5 8.5 Liquid Dispersant Acrylic acid/Acrylic acid/ Acrylic acid/ Acrylic acid/ Deionized Acrylic acid/additive methyl methyl methyl methyl water only methyl (500 g) acrylate= 3/1 acrylate = 10/0 acrylate = 3/1 acrylate = 10/0 acrylate = 3/1Weight 10,000 10,000 10,000 10,000 — 10, 000 average molecular weightConc. (wt %) 2 4 9 6 — 1 pH 7.5 6.5 7.9 6.0 7 7.3Plasma-CVD-TEOS-silicon 2,000 1,500 1,400 1,200 2,000 1,000 oxide filmpolishing rate (Å/min) Low pressure-CVD- 40 20 20 20 400 50 siliconnitride film polishing rate (Å/min) Polishing rate ratio 50 75 70 60 520 (silicon oxide film/ silicon nitride film) Flaws on polished oxide0.10 0.05 0.04 0.05 0.50 0.45 film (number/cm²) Degree of dishing (Å)140 70 130 60 850 170

As apparent from Table 1, the CMP abrasive and method for polishingsubstrates according to the invention can polish a surface to bepolished, such as a silicon oxide film or a silicon nitride film,without contaminating the surface to be polished with alkali metals,such as sodium ions, nor making flaws, and further can increase ratio of(silicon oxide film polishing rate)/(silicon nitride film polishingrate) to 50 or more.

INDUSTRIAL APPLICABILITY

The CMP abrasive of the invention is suitable for polishing methods usedin the production of semiconductor elements, particularly for polishingsubstrates for shallow-trench separation because it can speedily polisha surface to be polished, such as a silicon oxide film, without makingflaws.

The CMP abrasive of the invention is as well advantageous in that itdoes not contaminate the surface to be polished with alkali metals, suchas sodium ions, and can increase the ratio of (silicon oxide filmpolishing rate)/(silicon nitride film polishing rate).

The CMP abrasive of the invention is suitable for polishing methods usedin the production of semiconductor elements because it can improve thestorage stability of cerium oxide slurries.

The method for polishing substrates of the invention is suitablyapplicable in the production of semiconductor elements because it excelsin polishing speedily a surface to be polished, such as silicon oxidefilm, without making flaws.

1-7. (canceled)
 8. A liquid additive for CMP abrasive, comprising adispersant and water, wherein the liquid additive for CMP abrasive is tobe used in a method for polishing a substrate comprising holding asubstrate having, formed thereon, a film to be polished against apolishing pad of a polishing platen, followed by pressing, and movingthe substrate and the polishing platen while supplying a CMP abrasive inbetween the film to be polished and the polishing pad to thereby polishthe film to be polished; and wherein the CMP abrasive is prepared byseparately preparing, a cerium oxide slurry containing cerium oxideparticles, a dispersant and water, and the liquid additive for CMPabrasive containing the dispersant and water, and mixing, at the time ofthe polishing, the cerium oxide slurry and the liquid additive for CMPabrasive.
 9. The liquid additive for CMP abrasive of claim 8, whereinthe dispersant is a polymer dispersant selected from the groupconsisting of a polymer containing ammonium acrylate as a copolymerizedingredient, a polyammonium-acrylate and a polyamine-acrylate.
 10. Theliquid additive for CMP abrasive of claim 9, wherein the polymerdispersant has a weight average molecular weight of 100 to 50,000. 11.The liquid additive for CMP abrasive of claim 9, wherein the polymerdispersant has a molecular weight distribution (weight average molecularweight/number average molecular weight) of 1.005 to 1.300.
 12. Theliquid additive for CMP abrasive of claim 9, wherein the polymerdispersant contains 10 mol % or less of free ammonia or a free amine,which does not form a salt.
 13. The liquid additive for CMP abrasive ofclaim 8, which contains 1 to 10% by weight of the dispersant.
 14. Theliquid additive for CMP abrasive of claim 13, wherein the dispersant isa polyammonium-acrylate or a polyamine-acrylate.
 15. The liquid additivefor CMP abrasive of claim 14, wherein each of the polyammonium-acrylateand the polyamine-acrylate has a weight average molecular weight of1,000 to 100,000.
 16. The liquid additive for CMP abrasive of claim 15,wherein each of the polyammonium-acrylate and the polyamine-acrylate hasa molecular weight distribution (weight average molecular weight/numberaverage molecular weight) of 1.005 to 1.300.
 17. The liquid additive forCMP abrasive of claim 14, wherein each of the polyammonium-acrylate andthe polyamine-acrylate contains 10 mol % or less of free ammonia or afree amine, which does not form a salt.
 18. The liquid additive for CMPabrasive of claim 8, which is pH 4 to
 8. 19. The liquid additive for CMPabrasive of claim 8, which has a viscosity of 1.20 to 2.50 mPa·s.